Hemicelluloses are widely abundant polysaccharides in nature, representing about 20-35% of lignocellulosic biomass. Hemicelluloses are estimated to account for one-third of all renewable organic carbon available on earth. R. A. Prade, Biotechnology and genetic engineering reviews 1995, 13, 101; E. Sjöström, Wood Chemistry, Fundamentals and Applications, Academic Press, Inc., San Diego 1981. Often, hemicelluloses are found as organic wastes or byproducts of renewable forest and agricultural products. A. Ebringerova, Z. Hromadkova, Biotechnology and genetic engineering reviews, 1999, 16, 325; C. S. Badal, Ind. Microbiol. Biotechnol. 2003, 30, 279. For instance, in the pulp and papermaking process, significant quantities of hemicellulose are solubilized and burned for fuel value. Hemicelluloses have not yet found broad industrial applications similar to cellulose. S. Xaio-Feng, S. Run-Cang, Science of Food and Agriculture, 2004, 84(8), 800. The development of new, high-valued products based on hemicelluloses would significantly improve the economics associated with the utilization of lignocellulosic biomass. The chemical composition of hemicelluloses is related to cellulose, but its morphological structure is significantly different. Due to the heterogeneity of their chemical constituents, hemicelluloses in their natural state are generally considered to be non-crystalline and are branched polymers of low molecular weight with a degree of polymerization of 80-200. E. Sjöström, Wood Chemistry, Fundamentals and Applications, Academic Press, Inc., San Diego 1981. The most commonly existing sugars that constitute hemicelluloses are D-glucose, D-mannose, D-xylose, D-glucuronic acid, 4-O-methyl-D-glucuronic acid, and D-galacturonic acid. I. Gabrielii, P. Gatenholm, Applied Polymer Science, 1998, 69, 1661. The chemical composition and structure of hemicelluloses vary with plant species. The chemical modification of hemicelluloses presents a new avenue for the preparation of materials with special properties that can increase the applications for these biopolymers of abundance. Modifications that can accommodate or overcome issues with molecular weight disparity or variations in chemical composition are especially needed. For years, health care and medical textile industries have been attempting to find new sources of natural biomaterials with performance properties that are comparable with the major natural and synthetic materials currently used such as cotton or acrylics. Highly purified cotton and other natural fibers have significant cost issues associated with them. Synthetic materials are non-renewable and contribute to significant environmental issues such as disposal. Hemicellulose is an abundant biomaterial that has the potential to replace these materials.
Starch is another carbohydrate polymer that is widely abundant and readily available in a number of commercial forms. Starch typically occurs as semi-crystalline granules composed of amylopectin (branched polymer, 4000 glucose units) and amylose (linear polymer, 1000 glucose units). Both amylopectin and amylose are composed of α-1-4-glucosidic units. P. J. Jenkins, A. M. Donald, Biological Macromolecules, 1995, 17(6), 315. The ratio of amylose to amylopectin varies significantly and is in part determined by the origin of the starch. A typical corn starch has an amylose content of 75% whereas the amylose percentage for potato and for waxy maize is typically 82% and 0%, respectively. H. F. Zobel, Starch/Starke, 1988, 40, 44. The low cost and commercial availability of starch in the market attracts researchers attempting to develop new functional starch derivatives for industrial applications. The industrial applications of starch derivatives depend on the degree of substitution and type of functional groups along the main backbone of the starch polymer, its properties (gelatinization, crystallization, retrogradation, gel formation), and the amylose/amylopectin ratios (which depend on the source of extraction). P. J. Jenkins, A. M. Donald, Biological Macromolecules, 1995, 17(6), 315. One disadvantage of using starch relative to hemicellulose is that starch has great demand as a food product, whereas hemicellulose does not.
Chitin is an abundant naturally occurring polysaccharide with annual production very near the levels of cellulose. Chitin consists mainly of β-(1-4)-2-acetamido-2-deoxy-D-glucose units. Despite much recent research into its utilization, its strong intermolecular hydrogen bonding and poor solubility in common organic solvents have so far prevented widespread utilization of chitin. George A. F. Roberts, Chitin chemistry, Macmillan, London, 1992, 64. Chitosan is the N-deacetylated form of chitin that is obtained by alkaline treatment of chitin (e.g., treatment with sodium hydroxide (NaOH)) at high temperature. Chitosan is another carbohydrate-based polymer that is very abundant as a byproduct of the fishing industry, is widely available and is of strong research interest. I. M. Helander, E. L. Nurmiaho-Lassila, R. Ahvenainen, J. Rhoades, S. Roller, International Journal of Food Microbiology, 2001, 71(2), 235. Chitosan and its derivatives have become useful polysaccharides in the biomedical area because of their biocompatible, biodegradable, and non-toxic properties. K. Y. Lee, W. H. Park, W. S. Ha, J. Appl. Polym. Sci., 1997, 63, 425. The anti-microbial and antifungal activities of chitosan and chitosan derivatives have previously been described. K. F. El-Tahlawy, M. A. El-bendary, A. G. Elhendawy, S. M. Hudson, Carbohydrate Polymer, 2005, 60, 421; Sang-Hoon Lim, S. M. Hudson, Carbohydrate Polymers, 2004, 56(2), 227. Chitosan has been found to inhibit the growth of a wide variety of bacteria and fungi. Moreover, chitosan has several advantages over other types of disinfectants in that it possesses a higher antibacterial activity, broader spectra of activity, a higher killing rate, and lower toxicity toward mammalian cells.
Several mechanisms were proposed for the antimicrobial activity of chitosan: (1) the polycationic structure of chitosan may interact with the predominantly anionic components (lipopoly-saccharides and proteins of microorganism surface) of cell membranes resulting in changes in the membrane permeability that causes death of the cell by inducing leakage of intracellular components (I. M. Helander, E. L. Nurmiaho-Lassila, R. Ahvenainen, J. Rhoades, S. Roller, International Journal of Food Microbiology, 2001, 71(2), 235; M. Vaara, T. Vaara, Antimicrobiology agents Chemotherapeutant, 1983, 24, 114; H. Nikaido, Escherichia coli and Salmonella: cellular and molecular biology, American Society for Microbiology, Washington, D.C., 1996, 1, 29; S. H. Lim, S. M. Hudsen, Carbohydrate Research, 2004, 339, 313); (2) the chitosan on the surface of the cell can form a polymer membrane that prevents nutrients from entering the cell (X. Wang, Y. Du, H. Liu, Carbohydrate Polymers, 2004, 56, 21; L. Y. Zheng, J. F. Zhu, K. S. Sun, Materials Science and Engineering, 2000, 18, 22; H. Liu, Y. Du, J. Yang, H. Zhu, Carbohydrate Polymers, 2004, 55, 291); (3) the chitosan of lower molecular weight enters the cell, binds to DNA and inhibits RNA and protein synthesis (X. F. Liu, Y. L. Guan, D. Z. Yang, Z. Li, K. D. Yao, J. Appl. Polymer Sci. 2001, 29, 1324); and (4) since chitosan could adsorb electronegative substances in the cell and flocculate them, chitosan may disturb the physiological activities of the microorganism leading to death of the cells (L. Y. Zheng, J. F. Zhu, Carbohydrate Polymers, 2003, 54, 527).
The incorporation of carboxylic acid groups and antimicrobial activity into both hemicelluloses and starches (as well as other biopolymeric carbohydrates) is of interest in order to develop chemical and physical functionality in these materials. Natural polysaccharides with high carboxylic acid content are expected to have superior hydrophilic properties useful in applications such as absorbents.
Recently, new hemicellulose based superabsorbent materials have been synthesized by several scientists. A new hydrogel has been synthesized by graft copolymerization of 2-hydroxyethyl methacrylate (HEMA) or poly(ethylene glycol) dimethacrylate (PEGDMA, a cross linking agent) with oligomeric hydrosoluble hemicellulose modified with well-defined amounts of methacrylic functions. M. S. Lindblad, A.-C. Albertsson, E. Ranucci, ACS Symposium Series, 2004, 864 (Hemicelluloses), 347. The grafted copolymer was elastic, soft, and easily swellable in water. The viscoelastic and solution rheological properties of the grafted copolymer were characterized. Also, a comparison of hemicellulose-based hydrogels with pure poly(2-hydroxyethyl methacrylate) hydrogels showed that their behaviors were similar, demonstrating the potential of hemicellulose-based gels to compete with gels derived from petroleum based resources. However, the swelling properties of the hemicellulose/poly(hydroxyethyl methacrylate) were not adequate enough for many applications.
Hemicellulose (xylan) aspen wood has been separated with an alkali extraction method combined with ultrafiltration. I. Gabrielii, P. Gatenholm, W. G. Glasser, R. K. Jain, L. Kenne, Carbohydrate Polymers, 2000, 43(4), 367. The hemicellulose was sparingly soluble in cold water but soluble in hot water. Solutions of the hemicellulose did not exhibit good film forming properties. When mixed with chitosan, however, a gel was formed and films could be produced at compositions of 5% chitosan and above in acidic conditions. Ionic complexes between glucuronic acid functionalities of the hemicellulose and amino groups of chitosan were suggested to be responsible for network formation (interpolyelectrolyte complex). The morphologies of these films were examined, and a pure xylan film proved to be crystalline. The crystallinities were found to decrease with an increasing amount of chitosan, and the film of pure chitosan had virtually no crystallinity Films of mixtures of xylan with chitosan displayed slightly higher degrees of crystallinities than would be predicted from the weighted averages of the pure xylan and the pure chitosan films. When immersed in water, films with 5-20% chitosan formed hydrogels, and the degree of swelling of the hydrogels was shown to increase as the films contained more chitosan. Films with more than 20% chitosan dissolved in water. The film and hydrogel forming properties were attributed to crystalline domains of xylan interacting with the chitosan chains, as well as to electrostatic interactions between the acidic groups in the hemicellulose and the amino groups in the chitosan. In this study, the use of unmodified hemicellulose provided only a single, low carboxyl content of the hemicellulose which limited the ionic complexation between the carboxyl groups of hemicellulose and the amino group of chitosan (interpolyelectrolyte complex). This study, in addition to other studies, shows that bio-based materials have strong potential in the production of advanced gel materials.
Accordingly, there is a need for new, high-valued, industrially applicable, and environmentally efficient biomaterials based on hemicellulose, as well as other biopolymeric carbohydrates, that have exceptional performance properties.